The present invention generally relates to a method for processing and displaying seismic data.
It is conventional in seismic prospecting to place a plurality of seismic receivers along the earth's surface at spaced locations. A plurality of seismic sources disposed at spaced locations along the same line are then activated to produce seismic energy waves which spread out in all directions. Vibrating devices and explosive devices are common examples of such seismic sources. The seismic waves generated by the seismic source are reflected, refracted and defracted from subsurface interfaces in the earth, and some of these diverted waves are detected by the seismic receivers. The seismic receiver produces an electrical signal which can be processed to form a seismic signal. The seismic signals are displayed as seismic sections which contain information about the time, duration and intensity of the seismic wave. The seismic data displayed in such seismic sections can be studied to extrapolate information regarding the type and location of subsurface formations producing the received waves. This information in turn is employed to evaluate the subsurface formations for the oil and gas bearing properties.
Because of the geometry involved, seismic waves reflected from a common reflecting point can be received by a first seismic receiver from energy emitted by a first seismic source and also by a second seismic receiver from energy emitted by a second seismic source. These phenomena are employed in developing common depth point (CDP) seismic data. Because of the redundancy of the information obtained in this technique, seismic signals may be combined or averaged so that a high signal-to-noise ratio is obtainable. In common practice, from 3 to 48 sourcedetector pairs of seismic signals form a "gather" of CDP seismic signals. A plurality of CDP gathers can then be combined to form a seismic section.
The common depth point technique for processing and displaying seismic data is widely employed. The CDP method obtains multiple coverage of common subterranean reflection points. In CDP processing of seismic data, seismic signals representative of seismic energy having a common reflection point but which have penetrated the earth's subterranean formation along widely different ray paths and have different offset distances, are summed or stacked. Prior to summing or stacking, the seismic signals are first processed using the normal-move out technique to compensate for the different seismic energy ray paths and offset distances.
The CDP method enhances reflection events in the seismic signals which correspond to the assumed ray path and reduces other events. Enhanced reflection events in the seismic signal can then be plotted as a plurality of traces to form a seismic section which is a mapping of the reflectivity characteristics of the earth's subterranean formations. Additionally, random background noise developed in each seismic signal can be reduced by summing together the various seismic signals having a common reflection point.
Due to general inhomogeneities of the earth's subterranean formations, the CDP processing technique tends to mask or average lateral variations in the earth's subterranean formations encountered by the seismic wave traveling along different ray paths to a common reflection point. The CDP processing technique also tends to mask or average range dependent amplitude variations of the reflected signals. Attempts to compensate or ameliorate these masking or averaging effects have been proposed by others such as 0. E. Naess "Single-Trace Processing Using Iterative CDP-Stacking," Geophysical Prospecting 30, pages 641-652 (1982). Naess suggests that only the near traces of a CDP gather of seismic signals need be subjected to an iterative processing technique to produce a normal incident seismic section. Naess' iterative seismic section is in all respects an average of the near traces of CDP gather of seismic signals. The other traces in the CDP gather of seismic signals do not contribute to the final result except in determining which amplitudes on the original near traces of the CDP gather of seismic signals should be kept and which should be reduced, and then how much these should be reduced. As such, the primary information to be derived from Naess' iterative near trace seismic section was already present on the original near trace of CDP gather of seismic signals before the iterations were run. The iterations only effect a reduction in the noise of the original near traces of the CDP gather of seismic signals.
In a way this is what one attempts to achieve in conventional CDP stacking. One tries to convert all the traces of a CDP gather of seismic signals into near traces (i.e. zero offset seismic signals) through the expedient of normal move out corrections. That is, a stacking velocity is assumed for the propagating seismic wave so that common reflecting point events tend to align along a straight line rather than along a hyperbolic curve. Thereafter, all traces in the CDP gather of seismic signals are treated as equal and summed together. Unfortunately, all traces in a CDP gather of seismic signals are not equal since they result from encountering different subterranean formations due to the difference in the seismic wave ray paths combined with the general inhomogeneity of the earth. The difference in conventional CDP processing from that of Naess' is that Naess' iterative near trace seismic section treats the balance of the traces in a CDP gather of seismic signals merely as an aid in processing the near traces of a CDP gather of seismic signals, while in conventional CDP processing all the traces in a gather of seismic signals are treated as if they were actually near trace seismic signals.
That lateral variations in the earth's subterranean formations can have interpretive significance has been demonstrated by Ostrander in U.S. Pat. Nos. 4,316,267 and 4,316,268.